1. Field of the Invention
The present invention is in the field of implant materials. More particularly, the present invention is directed to compliant dehydrated implant materials which have no viable cells, and can be stored and transported without being immersed in liquid. The present invention is also directed to the process of producing said implant materials.
2. Brief Description of the Prior Art
The use of autografts, homografts and xenografts for augmenting or replacing defective tissues in humans and animals has been known for a long time. From the standpoint of providing suitable materials for implantation, augmenting or replacing hard tissues, such as bone, presents a different type of problem than augmenting or replacing soft tissues. In the selection of substitute materials for hard tissue graft, the strength and hardness of the graft are important whereas compliance and flexibility are, generally speaking, less crucial.
On the other hand, in the selection of soft tissue materials for implantation, compliance and flexibility of the graft material are usually of utmost importance because the soft tissue replacement material usually must closely match the healthy functional tissue that will be replaced. In this regard it must be remembered that natural soft tissue containing collagen is strong and able to withstand repeated three-dimensional stress as well as bending and deformation. Often natural soft tissue acts as a physical barrier that must maintain its structural integrity. Ideally, replacement or augmentation soft tissue that is utilized in implantation should have the same characteristics as the natural soft tissue that it replaces, and should be easy to obtain, store and transport. These, however are difficult goals that the prior art has been striving to attain, and up to the present invention only with moderate success.
More particularly, in accordance with one major approach in the prior art to preserve soft tissue for eventual implantation, tissues of human or animal origins have been treated with chemical modifiers/preservatives, such as glutaraldehyde, which cross-links collagen and other proteins. The glutaraldehyde treated tissues have been shown to be adequately resistant to mechanical fatigue as well as biodegradation when implanted in human patients. However, the glutaraldehyde cross-linking alters the viscoelastic properties of tissues, and therefore, as a result of host response undesirable calcification and build-up of peripheral granulation tissues usually occur in the implants with time. Glutaraldehyde is an effective biocidal (sterilyzing) agent, but when exposed to air it slowly loses its biocidal effectiveness. Therefore, the tissue intended for implantation (bioprosthesis) must be kept in glutaraldehyde solution during storage and transportation and the package including the glutaraldehyde soaked bioprosthesis must be kept tightly sealed. Moreover, it must not be exposed to significantly elevated temperature. Because of these requirements the costs of utilizing glutaraldehyde-treated soft tissue bioprostheses are high. Glutaraldehyde is toxic, and therefore it must be carefully removed from the bioprosthesis by rinsing before implantation. This represents still another disadvantage of glutaraldehyde-treated bioprostheses.
Another major approach for providing soft tissue bioprosthesis in the prior art utilizes liquid sterilants other than glutaraldehyde. Some of these alternative approaches also avoid the calcification problems associated with glutaraldehyde treated implants. However, in accordance with these processes also, to avoid brittleness and to more-or-less maintain the physical integrity of the bioprostheses the tissues have to be maintained, stored and transported in liquid media up to the time immediately preceding implantation.
Still another alternative method for providing soft tissue bioprostheses is the use of cryo-preserved fresh tissues of homograft (tissue from the same species). Because of recent advances in cryo-preservation, the cryo-preserved fresh tissues have recently made homograft implants relatively more successful and more accepted as an alternative to glutaraldehyde-preserved xenograft. A serious disadvantage of cryo-preserved bioprostheses is the difficulty to assure that they are free of infectious disease agents. The costs of preparing and handling of cryo-preserved bioprosthesis tissues is also very high because of the need for keeping the tissues at all times below the usual or normal freezer temperatures.
From among the numerous patent disclosures in the prior art directed to preparing and/or preserving biological tissue for implantation or other use as replacement tissue, U.S. Pat. Nos. 5,116,552 (Morita et al.) and 5,336,616 (Livesey et al.) are mentioned as of interest to the present invention. U.S. Pat. No. 5,116,552 (Morita et al.) describes a process for preparing lyophilized collagen sponge for medical applications, such as artificial skin. The process of this reference comprises the steps of impregnating cross-linked collagen sponge with an aqueous solution of a hydrophilic organic solvent, freezing the sponge and thereafter vacuum drying (lyophilizing) it. However, the resulting freeze-dried product is not pliable and is not protected from cracking because the water and the hydrophilic solvent or solvents have been removed in the lyophilization step. U.S. Pat. No. 5,336,616 (Livesey et al.) describes treatment of soft tissue obtained from a source, such as a cadaver, with solutions containing antioxidants, protease inhibitors and antibiotics (stabilizing solution), with enzymes and detergents to remove viable antigenic cells (processing solution), and after decellularization with a cryopreservative solution that prevents destructive ice crystal formation while the tissue is frozen. The cryo-preserving solution may include a combination of organic solvent and water. After lyophilization the product is stored and transported in a sealed container in an inert gas atmosphere, thus protected from atmospheric moisture. Prior to implantation the tissue is rehydrated and must be restored with immunotolerable viable cells to produce a permanently acceptable graft for implantation.
Still other disclosures pertaining to the preparation and/or preservation of biological tissue for implantation, or related subjects, can be found in U.S. Pat. Nos. 2,106,261; 2,610,625; 2,645,618; 3,939,260; 4,277,238; 4,280,954; 4,300,243; 4,383,832; 4,578,067; 4,703,108; 4,704,131; 4,760,131; 4,801,299; 4,911,915; 5,028,597; 5,131,850; 5,674,290 and U.K. Patent Specification 716,161.
It is an object of the present invention to provide a soft tissue graft suitable for implantation in humans or other mammals which graft after rehydration has substantially the same mechanical properties as the natural soft tissue from which the graft was obtained.
It is another object of the present invention to provide a soft tissue graft that satisfies the foregoing objective, that is also devoid of viable cells and does not require inoculation with viable cells prior to implantation.
It is still another object of the present invention to provide a soft tissue graft that satisfies the foregoing objectives, that can be stored and transported in a dehydrated form.
The foregoing and other objects and advantages are attained by a soft tissue preparation that in its dehydrated state is compliant, resists cracking, is devoid of viable cells and which is obtained by successively treating natural soft tissue:
with liquid compositions of gradually increasing concentrations of a C1-C3 alcohol, or other polar water miscible organic solvent in water, until the last of said liquid compositions contains at least approximately 25% by volume alcohol, or the other organic solvent, or mixtures thereof, the balance being water;
thereafter with a second liquid composition of aqueous glycerol or of low molecular weight ( less than 1000 D) polyethylene glycol, containing the glycerol or the low molecular weight polyethylene glycol, or mixtures thereof, in a concentration range of approximately 10 to 50% by volume, said second liquid composition also containing approximately 3-20% weight by volume polyethylene glycol of a molecular weight in the range of 6,000 D to 15,000 D and approximately 2 to 75 unit per milliliter heparin of a molecular weight greater than approximately 3 KD;
thereafter draining excess liquid from the soft tissue so treated;
thereafter immersing the soft tissue in an aqueous heparin solution of approximately 20 to 500 unit per milliliter concentration, and
thereafter freezing the tissue and lyophilizing the tissue to dryness.
The features of the present invention can be best understood together with further objects and advantages by reference to the following detailed description of specific examples and embodiments.